Endoluminal stents, particularly endovascular stents, are of considerable interest in the medical profession, especially to vascular surgeons. Such stents are presently used as a post-angioplasty adjunct to maintain the angioplasty-treated blood vessel in an open condition. Examples of endoluminal stents in the art include pressure-expandable stents which radially expand using a balloon angioplasty catheter, such as the Palmaz stent in U.S. Pat. No. 4,733,665; or self-expanding stents which radially expand due to the inherent spring tension of a wire material, such as the stent described by Gianturco in U.S. Pat. No. 4,580,568. Self-expanding stents which expand upon application of a stimulus, such as Nitinol stents or shape-memory polymer stents that expand when exposed to an increase in temperature, have also been described (Froix, U.S. Pat. No. 5,163,952).
In some applications it is desirable to cover the stent with a biocompatible material, since the stents themselves are often thrombogenic and the open nature of the stents can result in growth of tissue through the stent and into the lumen causing occlusion. It has also been desirable in some applications, and in particular for applications employing a metal stent, to provide a means for delivery of a therapeutic agent at the site of stent placement. One approach to meeting these desires has been to provide a polymer sleeve or sheath over the metal stent.
For example, in U.S. Pat. No. 5,674,241 to Bley et al. is described an expandable support stent having an expandable polymer layer coupled to the support stent. The expandable polymer layer is hydrophilic and expands upon hydration.
In U.S. Pat. No. 5,707,385 to Williams an expandable membrane having a drug reservoir is described. The membrane is mounted on the outer surface of a stent and the membrane-covered stent is delivered intraluminally to an injured or diseased site using a balloon-inflatable catheter.
U.S. Pat. No. 5,383,928 to Scott et al. discloses a sheath which encompasses a stent. The sheath serves as a local drug delivery device to prevent thrombosis and/or restenosis.
In U.S. Pat. No. 5,637,113 to Tartaglia et al. there is described an expandable stent structural member having a planar sheet of polymeric material attached to the outside of the structural stent member.
However, the polymer-stent combinations described heretofore in the art have a variety of shortcomings. For example, some polymer sheaths covering a metal support stent result in a significant reduction of flexibility and tractability of the stent, making deployment into a peripheral vascular network difficult. The reduction in flexibility and tractability also makes access past tortuous portions of a vessel difficult or impossible. Polymer membranes that are thick enough to carry a sufficient drug load can require a large balloon pressure to expand the stent and the coextensive polymer membrane into its open state. Large balloon pressures are undesirable for safety reasons. Thick polymer members also increase the profile of the overall stent limiting access to distal portions of the vessel. Additionally, many of the polymer membranes and sheaths axially shorten with radial expansion, leaving the ends of the metal stent uncovered.
There are other problems associated with polymer sleeves described in the art. Because the polymer sleeves are expanded to fit into a lumen, they are often made from a material having some elasticity. The polymer sleeve needs sufficient elasticity for expansion and for a snug fit about the support stent prior to expansion, but should exert little restoring force after expansion. Many polymer sleeves after expansion exert a recovery force on the support-stent, preventing the stent from remaining in its fully expanded state, and in the worst cases, can cause the support stent to collapse. Most often the recovery force exerted by the polymer sleeve or member causes the stent to partially recoil, thereby causing obstruction in the vessel lumen and setting up conditions that lead to thrombosis. On the other hand, if the polymer sleeve has insufficient elasticity, the sleeve can drape or sag through openings in the support stent after expansion in a lumen. This disrupts blood flow in the lumen and in severe cases reduces flow to a dangerous level or actually block blood flow altogether.
Also described in the art are polymer sleeves having no elasticity but which are wrapped or folded along the long axis of a support stent. Sufficient polymer material is wrapped about the support stent to allow for expansion from the small stent diameter during placement to the large expanded diameter after deployment. One problem associated with such polymer sleeves is in determining the amount of material needed to provide a uniform covering about the vessel lumen after expansion. To do this with any accuracy, the inner diameter of the target site lumen must be known, which is not always the case. Further, these polymer sleeves, because they are multiply wrapped about the support stent, often bunch-up during tracking to reach the target site, particularly when vessels having a smaller diameter than the target vessel must be navigated.
Accordingly, there is a need in the art for a polymer member designed to be carried on a support stent which overcomes these and other shortcomings.